A principal advantage asserted for internally illuminated roadway signs is that they can be seen at night even when out of reach of headlights of approaching vehicles. But this internal illumination also becomes a serious disadvantage when the light source within the sign partially or wholly fails. Under such circumstances the sign may become partially or wholly illegible or inconspicuous, especially to motorists traveling at highway speeds at night.
Others have suggested ways for imparting retroreflectivity to internally illuminated signs. For example, in U.S. Pat. No. 3,510,976, it is suggested that a sign face be formed by partially embedding, as a monolayer in an adhesive layer coated on a transparent plate, a mixture of metallized glass microspheres and non-metallized microspheres. The metal on the protruding portions of the metallized microspheres is then removed, a clear material coated over the microspheres, and an opaque sign image painted over the clear layer. The non-metallized microspheres are said to transmit light from the internal source, while the metallized microspheres retroreflect light beamed against the front of the sign.
A major difficulty with a sign as described in U.S. Pat. No. 3,510,976 is that light is poorly transmitted through the non-metallized microspheres, and thus the internal illumination of the sign is greatly reduced. Further, it would be expensive to make existing signs retroreflective using the technique taught in the patent, since that would require replacement of the sign faces. Insofar as is known, signs such as taught in U.S. Pat. No. 3,510,976 have never become commercial.